In producing sanitary ware as opposed to flat ceramic ware, the pre-fired casting is produced normally by a slip casting process wherein a slip is introduced into the mold and forms a body conforming to the internal shape of the mold. After a given time, the slip forms or casts into the desired shape and the remaining slip is removed or drained from the mold. A slip for sanitary ware is formed from a special coarse-grained clay or group of clays normally having a maximum grain size of about 20 microns. Maximum grain or particle size means about 90 percent of the particles are less than 20 microns. The coarse particle characteristic is determined by the amount of particles with a percentage finer than 5 microns, 2 microns, 1 micron and 0.5 micron. These finer grain size characteristics are a measurement of the coarse-grained clay or clays used in forming a slip. The clay or clays are combined with other constituents of the slip. The other constituents are normally kaolin, feldspar and silica, which is often ground silica. Use of a coarse-grained clay for the slip is standard practice because the resulting slip must have a desired rheological behavior together with a relatively high modulus of rupture (MOR) in the green condition. Furthermore, the slip must be such that the drain of the mold is maintained clean and the constituents of the slip must have a relatively low organic content. Low carbon content results in less defects in the glaze and increases the whiteness of the fired ceramic ware. It has been found that coarse-grained clays in the slip results in a desired casting process together with desired drying, glazing and firing attributes of the procedure for making sanitary ware. Because of the resultant characteristics mentioned above attributed to the use of coarse-grained clays having a relatively low amount of particles below 0.50 microns, the sanitary ware industry has heretofore uniformly used a coarse-grained clay or clays for the slip used in making sanitary ware.
At one time, it was suggested that the casting slip for sanitary ware casting should have high percentage of particle size above 45 microns, a substantially larger particle size than normally used for sanitary ware. Producing a casting slip having such larger particle size coarse-grained clay is disclosed in Thomas U.S. Pat. No. 7,105,466, which is incorporated by reference herein for general technical information. This patent describes procedures for increasing the particle size of the coarse-grained clays used for casting slips to cast sanitary ware. Indeed, several general characteristics of a casting slip used for sanitary ware is described in this patent, even though it is directed to the irrelevant concept of using larger particle size coarse-grained clay for the slip. This patent does not describe background for the present invention; but, it does explain certain terms and characteristics of casting slips used in sanitary ware. The clay or clays of this prior patent are not the traditional clay used in sanitary ware production, but a clay blend having even larger particle size. The clay used for sanitary ware and to which the present invention is directed is different. The ball clay forming the background of the invention has a limited maximum particle size and less than about 30 percent of its particles with the particle size less than 0.5 microns. This type clay is the coarse-grained clay used in sanitary ware production and forming the background of the present invention. Such coarse-grained clay is traditionally used for making sanitary ware is a clay of the type sold by Unimin Corporation of New Canaan, Conn. under the trademark STARCAST XL. The slip for sanitary ware uses this type of coarse-grained ball clay together with kaolin, feldspar and silica to produce a traditional slip for making sanitary ware. Thus, this type slip constitutes the technical background to the present invention. The invention is not related to increasing the maximum particle size of the traditional coarse-grained ball clay as suggested in Thomas U.S. Pat. No. 7,105,466. To the contrary, the present invention modifying the traditional coarse-grained ceramic casting ball clay used in producing sanitary ware blended by adding a “pressing” clay. This modifying clay results in a clay blend that increases the strength of the casting and decreases the cost of the slip without substantially affecting the casting characteristics of the prior slip using traditional coarse-grained clay for the specific purpose of producing sanitary ware.